U.S. Pat. No. 4,510,489, issued to Anderson et al., discloses a magnetomechanical electronic article surveillance (EAS) system in which markers incorporating a magnetostrictive active element are secured to articles to be protected from theft. The active elements are formed of a soft magnetic material, and the markers also include a control element which is biased or magnetized to a pre-determined degree so as to provide a bias field which causes the active element to be mechanically resonant at a pre-determined frequency. The markers are detected by means of an interrogation signal generating device which generates an alternating magnetic field at the pre-determined resonant frequency, and the signal resulting from the mechanical resonance is detected by receiving equipment.
According to one embodiment disclosed in the Anderson et al. patent, the interrogation signal is turned on and off, or "pulsed," and a "ring-down" signal generated by the active element after conclusion of each interrogation signal pulse is detected.
Typically, magnetomechanical markers are deactivated by degaussing the control element, so that the bias field is removed from the active element thereby causing a substantial shift in the resonant frequency of the active element.
The Anderson et al. patent discloses a number of materials that may be used for the active element, and also describes techniques used for treating the materials. The disclosed techniques include heat-treating (annealing) an amorphous material in a saturating magnetic field. The disclosure of the Anderson et al. patent is incorporated herein by reference.
U.S. Pat. No. 5,252,144, issued to Martis, discloses further materials said to be suitable for use as active elements in magnetomechanical EAS markers, as well as annealing processes (without application of a magnetic field) to be applied to the materials.
The above-referenced '651 co-pending application discloses a procedure in which batches of pre-cut strips of an amorphous metal alloy are annealed in the presence of a saturating transverse magnetic field. The resulting annealed strips are suitable for use as the active elements in magnetomechanical markers and have improved ring-down characteristics which enhance performance in pulsed magnetomechanical EAS systems. In addition, the hysteresis loop characteristic of the resulting active element is such as to eliminate or reduce false alarms that may result from exposure to harmonic EAS systems. Moreover, the procedure disclosed in the '651 application produces active elements that are relatively flat in longitudinal profile, permitting fabrication of quite thin markers incorporating such active elements. The disclosure of the aforesaid application Ser. No. 08/269,651 is incorporated herein by reference.
The above-referenced '757 co-pending application discloses an adaptation of the techniques of the '651 application, in which a continuous process is employed to transport a continuous ribbon of amorphous metal alloy from reel to reel through an oven in which transverse-field annealing is carried out. Then, after annealing, the continuous ribbon is cut into discrete strips. This continuous annealing process avoids inconvenience in transporting pre-cut strips into and out of an oven.
The techniques disclosed in the '651 and '757 copending applications represent advances over previously known techniques. However, it would be desirable to modify the techniques of those two co-pending applications so as to provide active elements for EAS markers having a resonant frequency that is relatively insensitive to variations in the biasing magnetic field.